It is known that a hydration phenomenon as described below occurs depending on a solute in aqueous solution. In a case where the solute is an electrolyte like NaCl, a hydration phenomenon in which a water molecule is constrained by the solute occurs due to electrolytic dissociation of the solute to ion. In a case where the solute is a nonelectrolyte like sugar, a hydration phenomenon occurs through electrostatic force or hydrogen bonding that is caused by a deviation of polarities in solute molecules. A hydration phenomenon is also largely involved in activity of a macromolecule such as protein.
In aqueous solution, when a water molecule is replaced with protein, bulk water (water in which a water molecule is sufficiently away from, a solute and not bound with protein) is reduced, so that a dielectric constant of the bulk water changes to a dielectric constant of the protein. In a complex dielectric constant of bulk water illustrated in FIG. 10 (FIG. 2 of NPL 3), in particular, an imaginary part of the complex dielectric constant largely varies in a frequency region particularly in a vicinity of 100 GHz due to a relaxation phenomenon of the bulk water. Since a main component of a living body is water, by checking not only a real part but also the imaginary part of the complex dielectric constant in a frequency region in a vicinity of 100 GHz, it is possible to check states of the living body and a biopolymer.
As a technique of detecting a change in a dielectric constant in a high frequency region, a sensor device 101 as illustrated in FIG. 11 is known as a related art (for example, NPLs 1 and 2). The sensor device 101 is formed on an integrated circuit and includes an oscillation unit 102 and an oscillation frequency detection unit 103. The oscillation unit 102 is constituted by a resistor R1, transistors M1 and M2 which are cross-coupled to each other, and a resonator 104. The resonator 104 is constituted by inductors L1 and L2, two sensing electrodes 105 that are made in contact with an inspection object 100, and a capacitor C3. A resonant frequency of the resonator 104 is 6 to 30 GHz.
As illustrated in FIG. 12, the two sensing electrodes 105 are constituted by two plate electrodes 111 and 112 each of which has a rectangular shape. As illustrated in FIG. 13, the plate electrodes 111 and 112 are formed on an uppermost metal wiring layer of a semiconductor integrated circuit. FIG. 13 illustrates structures of the plate electrodes 111 and 112 in a cross section taken along a line A-A in FIG. 12 and viewed in the direction of the arrows. An interlayer insulating film 115 is disposed between metal wiring layers of the semiconductor integrated circuit. For convenience, FIG. 13 illustrates only the uppermost metal wiring layer and its lower interlayer insulating film 115. A surface protection film 114 covers a surface of the interlayer insulating film 115, but the surface protection film 114 does not cover regions thereof where the two plate electrodes 111 and 112 are disposed. Thus, exposed upper surfaces of the plate electrodes 111 and 112 directly contact the inspection object 100.
Next, an operation of the sensor device 101 will be described. In a case where a dielectric constant of the inspection object 100 that is near the sensing electrodes 105 changes, a parasitic capacitance value to the sensing electrodes 105 changes and the resonant frequency of the resonator 104 changes. A change in an oscillation frequency of the oscillation unit 102, which is associated with the change in the resonant frequency, is detected by the oscillation frequency detection unit 103. With the operation above, the sensor device 101 is able to detect the change in the dielectric constant, which is generated in the inspection object 100 near the sensing electrodes 105, as the change in the oscillation frequency.